Interpretive Summary: Chicken eggs for hatching purposes are collected from hens from about 25 weeks of age until about one year of age. It is generally thought that as a hen ages she lays eggs with thinner and lower quality shells. Salmonella and other bacteria can penetrate eggshells and once within an egg can cause spread of this human pathogen to many chicks. Such contamination can be carried to the processing plant and final product, presenting a human health hazard. This study was done to determine if shell quality or hen age is related to the ability of Salmonella to penetrate the shell. Eggs were examined at monthly intervals across the productive period of a commercial breeder flock. Eggshell quality measures related to the thickness and porosity of the shell were not correlated with Salmonella penetration. Furthermore, there was no clear relationship between hen age and Salmonella penetration of the eggshell. Factors other than eggshell quality may be involved in bacterial penetration of chicken eggs.

Technical Abstract:
Egg weight, specific gravity, conductance and ability of Salmonella to penetrate the shell and membranes were determined for hatching eggs from a commercial broiler breeder flock. Thirty eggs were sampled on weeks: 29, 34, 39, 42, 48, 52 and 56 of flock age for specific gravity and conductance. An additional 10 intact eggs were inoculated with Salmonella by a temperature differential immersion for one minute. Eggs were then emptied of contents and filled with a selective medium which allowed visualization of Salmonella growth inside the shell and membrane complex. Over the 25-wk sampling period, egg weight increased from 56 to 66 g and was positively correlated with hen age (r=.96, P<.05). However, neither specific gravity (ranging from 1.077 to 1.082) nor eggshell conductance (ranging from 14.7 to 17.9 mg/d/torr) showed any clear trend throughout the life of the flock despite the increase in egg weight. Conductance values were not correlated with specific gravity. The number of eggs positive for Salmonella penetration after 24 h incubation showed a general upward trend with flock age, however penetration frequency and hen age were not found to be significantly correlated (P>.05). No relationship was found between egg specific gravity, conductance or egg weight and the likelihood of Salmonella to penetrate the eggshell. Since shell characteristics did not change over time and the penetration patterns did vary, it is likely that factors other than specific gravity and conductance were involved in the penetration of eggshells by Salmonella.